This invention relates generally to an apparatus for fixing toners or the like on a receiving medium. The fixing is accomplished by pressure and/or heat being applied to the receiving medium as it and a toned image contained thereon pass through a nip formed by a pair of belts.
As in any apparatus for fixing toners, the feeding and contact parameters, as the receiving medium passes through the nip, are critical. Without the proper feeding and contact parameters the following problems may result:
1. Defects in the fixed image caused by localized stresses, velocity changes and static charge in the nip mechanics during the fixing process. This results from roller non-compliancy and the receiving medium traveling in multiple planes as it approaches, travels through and leaves the nip. Additionally, since solid or hard rollers, normally used in the fixing process, do not have the capacity to adapt between the thicker areas on the receiving medium, i.e., those containing the copy image, and the thinner areas, i.e., those not containing copy image; localized stress is further increased.
2. Internally heated rollers with inefficient thermal conductivity because their heat lamps are normally located within poor heat conductors such as thick elastomer or foam material rollers.
3. Distortions in the fixed image, paper grab, paper curl, loss of paper stiffness and excessive roller wear resulting from the increase in torque required for one roller to break the plane of the other roller when the nip width is increased by increasing the pressure in the nip.
In the past, to avoid feeding and contact problems, such as stated above, and to prevent movement of toner particles and localized stress that causes image defects and distortion, one tried to increase the compliancy of one or more of the rollers in the fixing assembly, the width of the nip or the flatness of the fixing nip to lessen the effects of such process-related defects and distortions. Attempts made to increase the compliancy, flatness or width of the nip and thereby lessen the fixing related defects included making the elastomeric layers of the rollers thicker and using elastomeric foam materials such as disclosed in U.S. Pat. No. 4,814,819. However, since fixing rollers are usually heated internally, it was difficult to obtain an elastomeric material that was sufficiently compliant while having a heat conductivity that efficiently provided the heat necessary to fix the toned images. Additional solutions, such as disclosed in Japanese Patent No. 61-90180, included forming a heated roller and a pressure roller of elastically deformable thin cylinders with a plurality of guide members arranged on the inside or outside of the cylinders. In that type of arrangement, however, it was difficult to maintain a uniform nip pressure, since the cylinders did not uniformly deform when a receiving medium entered the nip formed by such cylinders as shown in U.S. Pat. No. 4,563,073. Therefore, the above attempted solutions proved unsatisfactory.